Method for processing member having excellent chemical conversion treatability

ABSTRACT

In a method for processing a high-strength member having a high Si content of more than 0.7% by mass, a high-strength steel material containing, on the basis of mass percent, 0.05% or more C, more than 0.7% Si, and 0.8% or more Mn is subjected to processing controlled such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 5% or more as nominal strain to form a member having a predetermined shape. The member, even containing more than 0.7% Si, can have markedly improved chemical conversion treatability without mechanical grinding or chemical pickling treatment.

FIELD OF THE INVENTION

The present invention relates to a method for processing high-strength steel for auto body structures and, more particularly, to a method for improving the chemical conversion treatability of a member manufactured by the processing of a high-strength steel material having a high Si content of more than 0.7% by mass.

BACKGROUND OF THE INVENTION

In recent years, from the standpoint of global environmental protection, efforts have been made to reduce the weight of automotive bodies and improve the mileage of automobiles. The improvement in the mileage of automobiles has also been required by law. Recently, efforts have been made to use high-strength steel materials as the materials for automotive body structure to reduce the weight of automobiles by gauge down (thickness reduction). Furthermore, improvement in the stiffness of members using a closed-cross-section structure like a steel tube is under study.

In general, such high-strength steel materials are basically designed to contain 0.7% by mass or more Si to achieve both high strength and excellent formability. However, the inclusion of Si is inevitably accompanied with a marked deterioration in chemical conversion treatability. The mechanism of deterioration in the chemical conversion treatability of steel materials having a high Si content is known to some extent as described below.

In steel materials containing Si, an oxide mainly composed of Si is concentrated on a surface layer of the steel material (other equivalent expressions, such as a Si-based oxide, a Si-containing oxide, a Si oxide, and a Si group oxide, mean the same oxide; unless otherwise specified, these are collectively referred to as an oxide mainly composed of Si). An oxide mainly composed of Si inhibits the formation of iron-zinc phosphate crystals (Zn₂Fe(PO₄)₂.4H₂O) in the anode reaction and cathode reaction during chemical conversion treatment. Thus, dense and fine iron-zinc phosphate crystals cannot be formed on the steel material. As illustrated in FIG. 1, the chemical conversion treatment of a steel material having a high Si content results in the formation of iron-zinc phosphate crystals having coarse and sparse iron-zinc phosphate crystal free areas (hereinafter referred to as crystal-free areas). In contrast, as illustrated in FIG. 2, the chemical conversion treatment of mild steel having a low Si content (JIS-SPCC-grade steel sheets) forms very dense iron-zinc phosphate crystals.

In cold-rolled steel sheets, pickling of a hot-rolled steel sheet before cold rolling can partly remove an oxide mainly composed of Si. However, in cold-rolled steel sheets subjected to an annealing process, such as continuous annealing or batch annealing, after cold rolling, an oxide mainly composed of Si is again inevitably concentrated on a surface layer in a furnace even at a very low dew point. Thus, cold-rolled steel sheets also often have poor chemical conversion treatability. Furthermore, in the annealing process, gradual variations in the environment within the furnace, variations in the components of steel, or variations in manufacturing conditions often result in variations in the distribution of an oxide mainly composed of Si on the steel sheet. In the formation of an oxide mainly composed of Si, variations in the components of steel, variations in manufacturing conditions, and the like intricately interact with one another. It is therefore difficult to manage these influencing factors to control chemical conversion treatability.

Thus, the surfaces of steel materials (steel sheets) manufactured have hitherto been ground in a mechanical process or dissolved in a chemical process, such as pickling, to remove an oxide mainly composed of Si that inhibits chemical conversion. For example, PTL 1 describes a method for manufacturing high-tensile steel sheets with a high Si content having excellent phosphate coating treatability. This method includes annealing in an atmosphere in which the oxygen partial pressure is controlled within a particular range, quenching in a particular temperature range, grinding of the surface, and pickling to remove an oxide film.

PTL 3 describes a method for manufacturing high-strength cold-rolled steel sheets having excellent chemical conversion treatability. This method includes softening and annealing of cold-rolled steel sheets having a Si content/Mn content of 0.4 or more in an atmosphere at a dew point in the range of −20° C. to 0° C. such that the fraction of surface coverage of a Si group oxide is 20% or less and the equivalent circular diameter of the Si group oxide is 5 μm or less, water quenching, tempering, and immersion in hydrochloric acid or sulfuric acid for pickling.

PTL 12 describes a method for manufacturing high-strength electric-resistance-welded steel tubes having excellent chemical conversion treatability. This method includes hot-rolling and pickling of a steel sheet having a composition of Si: 0.5% by mass or less, Mn: 1.5% by mass or less, and P: 0.05% by mass or less to remove an outer surface layer and an inner surface layer, cold rolling at a cold-rolling reduction in the range of 10% to 60%, and electric-resistance welding (ERW) of both ends of the cold-rolled steel strip in the width direction to form a welded steel tube.

However, grinding or pickling requires a large number of man-hours, and it is difficult to completely remove an oxide mainly composed of Si. Furthermore, an oxide mainly composed of Si is glass and consequently does not dissolve in a common acid, such as hydrochloric acid or sulfuric acid. Furthermore, since an oxide mainly composed of Si cannot be selectively removed by pickling, a base steel sheet must be significantly dissolved to remove the oxide mainly composed of Si.

PTL 2 describes a method for treating a steel surface, which includes immersion of a steel material in a mixed acid of sulfuric acid and hydrofluoric acid at a sulfate ion concentration and a hydrogen fluoride concentration in particular ranges and subsequent immersion of the steel material in hydrochloric acid at a chloride ion concentration in a particular range. Although pickling in a fluorinated acid type agent can completely remove an oxide mainly composed of Si, use of the fluorinated acid type agent may somewhat increase the degree of danger.

PTLs 4 to 8 describe a technique for improving chemical conversion treatability by forming a Si—Mn composite oxide easily soluble in an acid while preventing the formation of a slightly soluble oxide mainly composed of Si.

PTL 4 describes a multiphase steel sheet having excellent coating adhesion in which the Si and Mn contents are controlled so as to satisfy a Si/Mn ratio of 0.4 or less, there are 10 or more fine Mn—Si composite oxide particles containing 0.5% by mass or more (Mn—Si) on a surface layer (an area 2 μm in depth and 10 μm in length), and an oxide mainly composed of Si accounts for 10% or less of the surface length of the steel sheet.

PTL 5 describes a multiphase high-strength cold-rolled steel sheet having excellent coating adhesion in which the Si and Mn contents are controlled so as to satisfy a Si/Mn ratio of 0.4 or less, there are 10/100 μm² or more fine Mn—Si composite oxide having a Mn/Si ratio of 0.5 or more, the fraction of surface coverage of an oxide mainly composed of Si is 10% or less, and there is no crack having a size in a predetermined range.

PTL 6 describes a multiphase high-strength cold-rolled steel sheet having excellent strength-elongation balance, that is, a high elongation/strength ratio, wherein the Si and Mn contents are controlled so as to satisfy a Si/Mn ratio of 0.4 or less, there are 10/100 μm² or more fine Mn—Si composite oxide having a Mn/Si ratio of 0.5 or more, the fraction of surface coverage of an oxide mainly composed of Si is 10% or less, and the tensile strength is 390 MPa or more.

PTL 7 describes a high-strength steel sheet having excellent coating adhesion in which the average distance between the starting points of Si- and/or Mn-containing oxide stemming from a surface of the steel sheet in the depth direction in a network-like or hair-root-like manner is 5 μm or more, and the total length of the oxide is 10 μm/(12 μm in depth×20 μm in width) or less.

PTL 8 describes a Si—Mn oxide multiphase high-strength steel sheet having excellent coating adhesion in which the Si and Mn contents are controlled so as to satisfy a Si/Mn ratio of 0.4 or less, there are 10/100 μm² or more fine Si—Mn oxide on the surface, and the fraction of surface coverage of an oxide mainly composed of Si is 10% or less.

Although a Si—Mn composite oxide adversely affects chemical conversion treatability as with an oxide mainly composed of Si, the Si—Mn composite oxide easily dissolves in an acid. In the techniques described in PTLs 4 to 8, therefore, a Si—Mn composite oxide is intended to be removed by “in-line pickling”, which is often provided in the production lines of cold-rolled steel sheets. However, in the techniques described in PTLs 4 to 8, since the Mn content depends on the Si content, there is a problem of a limited degree of freedom in the design of steel components. There is also a problem that improvement in chemical conversion treatability is often limited.

It is known that a zinc-phosphate-treated film for use in mechanical lubrication, which can be used in combination with a lubricant to facilitate plastic working, can be subjected to shot blasting as pretreatment to improve chemical conversion treatability. For example, PTL 9 describes a method for forming a conversion coating on a surface. The method includes ejecting a zinc phosphate chemical conversion treatment liquid to which silica sand has been added against the surface to clean the surface and then ejecting the zinc phosphate chemical conversion treatment liquid. It is assumed that the mechanism by which shot blasting before chemical conversion treatment can improve chemical conversion treatability is due to the mechanochemical activation of a surface by shot blasting (see NPL 1). However, leaving a shot-blasted surface to stand in the air or annealing a shot-blasted surface reduces the mechanochemical activity of the surface, failing to achieve a desired improvement in chemical conversion treatability.

Even when shot blasting is employed as pretreatment of coating, a considerable amount of time elapses from the shot blasting to coating in actual operation. The effects of improving chemical conversion treatability in actual operation are therefore markedly reduced and are not thought to be significant. The employment of continuous in-line shot blasting to reduce the time elapsed from shot blasting to coating requires considerable costs and therefore has a low degree of realizability.

PTL 10 describes a high-tensile hot-rolled steel sheet having excellent chemical conversion treatability and corrosion resistance, wherein the steel sheet contains 0.5% to 2.5% by mass Si and contains C and Ti such that C and Ti satisfy a particular relationship, the average grain diameter is 3.0 μm or less, and the surface roughness is controlled to 1.5 μm or less as an arithmetical mean roughness Ra. In accordance with the technique described in PTL 10, the small crystal grain diameter and the smooth surface result in a marked improvement in chemical conversion treatability.

NPL 2 has reported that the surface roughness of a steel sheet does not significantly affect chemical conversion treatability at Ra in the range of 0.5 to 1.7 μm, PPI in the range of 110 to 250, or Wz in the range of 1 to 8 μm.

PTL 11 describes a method for manufacturing cold-rolled steel sheets that can effectively improve phosphate treatability without impairing the press formability of the steel sheets. The method includes annealing of a steel sheet containing 0.01% by mass or less C, 0.01% by mass or less N, and Ti and skin pass rolling at a rolling reduction of 0.8% or more and 5% or less. In accordance with PTL 11, the chemical conversion treatability is saturated at a rolling reduction of 2.7% or more in the skin pass rolling.

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.     2003-226920 -   PTL 2: Japanese Unexamined Patent Application Publication No.     2004-256896 -   PTL 3: Japanese Unexamined Patent Application Publication No.     2004-323969 -   PTL 4: Japanese Unexamined Patent Application Publication No.     2005-248281 -   PTL 5: Japanese Unexamined Patent Application Publication No.     2005-281787 -   PTL 6: Japanese Unexamined Patent Application Publication No.     2005-290440 -   PTL 7: Japanese Unexamined Patent Application Publication No.     2006-144106 -   PTL 8: Japanese Unexamined Patent Application Publication No.     2005-187863 -   PTL 9: Japanese Examined Patent Application Publication No. 46-6327 -   PTL 10: Japanese Unexamined Patent Application Publication No.     2002-226944 -   PTL 11: Japanese Unexamined Patent Application Publication No.     62-116723 -   PTL 12: Japanese Unexamined Patent Application Publication No.     2004-292926

Non-Patent Literature

-   NPL 1: Tamai and Mori, Kinzoku Hyomen Gijutsu (The Journal of the     Metal Finishing Society of Japan), vol. 31, (1980), pp. 482-486. -   NPL 2: Suda et al., Tetsu To Hagane (Bulletin of the Iron and Steel     Institute of Japan), vol. 66, (1980), pp. S1130.

SUMMARY OF THE INVENTION

Marketing steel sheets and other products are subjected to stamping or bending to manufacture members. Thus, the surface qualities of press dies may be transferred onto the surfaces of the steel sheets and other products. Furthermore, the steel sheets and other products may be deformed. The original surface qualities are therefore rarely maintained. Thus, it is difficult to think that steel sheets manufactured by the techniques described in PTLs 10 and 11 always have excellent chemical conversion treatability even after processing.

Since skin pass rolling results in hardening, the skin pass rolling of higher-strength materials will gradually become more difficult. The skin pass rolling of steel materials having a tensile strength on the order of 780 MPa or more is difficult to perform at a rolling reduction of 1% or more. The skin pass rolling of steel materials having a tensile strength on the order of 590 MPa may be performed at a rolling reduction of no more than approximately 2%. Thus, the technique described in PTL 11 in which skin pass rolling is performed at a rolling reduction of 0.8% or more and 5% or less cannot be applied to high-strength materials without causing problems.

Thus, the truth of the matter is that the related art described above cannot significantly improve the chemical conversion treatability of steel materials having a high Si content of more than 0.7% by mass.

In view of the situations described above, it is advantageous to provide a method for processing members in which a high-strength steel material having poor chemical conversion treatability used as a raw material is processed into a high-strength member having excellent chemical conversion treatability.

More particularly, it is advantageous to improve the chemical conversion treatability of a high-strength member particularly using a high-strength steel material having a high Si content of more than 0.7% by mass in which an oxide mainly composed of Si is concentrated on a surface layer as in hot-rolled sheets or cold-rolled and annealed sheets. The steel materials include steel sheets (steel strips), steel tubes, and steel bars. The concentration of an oxide mainly composed of Si, as used herein, refers to the concentration of an oxide mainly composed of Si or an oxide containing Si and another element or the concentration of a composite oxide, a eutectic oxide, a peritectic oxide, or the like containing these.

To these ends, the present inventors have performed diligent research on various factors affecting the chemical conversion treatability of high-strength steel materials having a high Si content. As a result, the present inventors have conceived the utilization of processing strain applied to a surface during the processing of steel materials. The present inventors found that in the processing of steel materials into members the chemical conversion treatability of a member manufactured from a high-strength steel material having a high Si content can be significantly improved when the processing conditions are controlled such that the processing strain applied to a surface of the steel material (the surface strain) is a predetermined value or more. The present inventors also found that the improvement in chemical conversion treatability can be well explained by employing, as the processing strain, the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing. The present inventors also found that the chemical conversion treatability of a member made of a steel material that contains more than 0.7% of Si and in which an oxide mainly composed of Si is concentrated on a surface layer can be significantly improved by processing the steel material such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 5% or more as nominal strain.

The mechanism by which even the chemical conversion treatability of a steel sheet containing an oxide mainly composed of Si concentrated on a surface can be improved by applying a surface strain of 5.0% or more to the surface is not fully elucidated. The following is a possible mechanism.

It has often been pointed out that an oxide mainly composed of Si in a film form is concentrated on a surface of a steel sheet having a high Si composition. In the actual production using a continuous annealing line (CAL), an oxide mainly composed of Si is concentrated mostly in a granular form, for example, by in-line light pickling. In both cases, it is assumed that a granular oxide mainly composed of Si can be very easily removed (fell out) from the surface of the steel sheet by chemical conversion treatment under the surface strain of a predetermined value or more.

The present invention has been accomplished on the basis of these findings after further consideration. The exemplary aspects of the present invention are as follows:

(1) A method for processing a member having excellent chemical conversion treatability, wherein a high-strength steel material having a composition containing, on the basis of mass percent, 0.05% or more C, more than 0.7% Si, and 0.8% or more Mn, preferably further containing 0.1% or less Al and 0.010% or less N, or further containing one or at least two selected from 0.03% or less Ti, 0.1% or less Nb, and 0.1% or less V, and/or one or at least two selected from 1% or less Cr, 1% or less Mo, 1% or less Ni, 1% or less Cu, and 0.01% or less B, and/or one or two selected from 0.1% or less Ca and 0.05% or less REM, and containing a remainder of Fe and incidental impurities is subjected to processing to form the member, and the processing is controlled such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 5% or more as nominal strain.

(2) The method for processing a member according to (1), wherein the sum total of absolute surface strains in a predetermined direction is the sum total of absolute surface strains applied in two directions intersecting at right angles.

(3) The method for processing a member according to (1) or (2), wherein conditions for the processing are controlled such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing in combination with the sum total of absolute surface strains applied in the manufacture of the high-strength steel material is 5% or more as nominal strain.

(4) The method for processing a member according to any one of (1) to (3), wherein the steel material is a hot-rolled material or a cold-rolled material.

(5) The method for processing a member according to any one of (1) to (4), wherein the composition contains, on the basis of mass percent, 0.05% or more C, 1% or more Si, and 1.5% or more Mn, preferably further contains 0.1% or less Al and 0.010% or less N, or further contains one or at least two selected from 0.03% or less Ti, 0.1% or less Nb, and 0.1% or less V, and/or one or at least two selected from 1% or less Cr, 1% or less Mo, 1% or less Ni, 1% or less Cu, and 0.01% or less B, and/or one or two selected from 0.1% or less Ca and 0.05% or less REM, and contains a remainder of Fe and incidental impurities.

(6) The method for processing a member according to any one of (1) to (5), wherein the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 7% or more as nominal strain.

In accordance with exemplary embodiments of the present invention, a member manufactured using a high-strength steel material having a high Si content of more than 0.7% on the basis of mass percent as a raw material can be a high-strength member having excellent chemical conversion treatability without performing mechanical grinding or chemical pickling treatment. Thus, the present invention can have significant industrial advantages. Also in accordance with exemplary embodiments of the present invention, a member having excellent chemical conversion treatability can be manufactured independently of the history of a steel material used as a raw material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron microscope photograph of a surface structure after the chemical conversion treatment of high Si steel.

FIG. 2 is a scanning electron microscope photograph of a surface structure after the chemical conversion treatment of mild steel.

FIG. 3 is an explanatory drawing of an example of scribed circles 6.

FIG. 4 is a schematic explanatory drawing illustrating a processing method in a processing pattern a.

FIG. 5 is a schematic explanatory drawing illustrating a processing method in a processing pattern b.

FIG. 6 is a schematic explanatory drawing illustrating the shape of a member processed in a processing pattern c1.

FIG. 7 is a schematic explanatory drawing illustrating the shape of a member processed in a processing pattern c2.

FIG. 8 is a schematic explanatory drawing illustrating an SDT test method by which the corrosion resistance of a coating film after coating is evaluated.

FIG. 9 is an explanatory drawing illustrating an example of manufacturing facilities suitable for the manufacture of a welded steel tube according to the present invention.

DESCRIPTION OF EMBODIMENTS

In exemplary aspects of the present invention, a high-strength steel material having a high Si content of more than 0.7% by mass used as a raw material is processed into a high-strength member. The “high-strength” steel material, as used herein, refers to a steel material having a tensile strength of 590 MPa or more. The steel materials include steel sheets (steel strips), steel tubes, and steel bars.

Preferred compositions of a high-strength steel material used as a raw material will be described below. Unless otherwise specified, the percent by mass is denoted simply by %.

C, 0.05% or more

C is an element that can increase the strength of steel. The C content of 0.05% or more is required to ensure a high tensile strength of 590 MPa or more. Thus, the C content is limited to 0.05% or more. More than 1.0% C results in low ductility. In applications requiring weldability, the C content is preferably 0.5% or less. More than 0.5% C results in low integrity of a weld and low toughness. Thus, the C content is preferably 0.5% or less, more preferably 0.3% or less. C has a very small influence on chemical conversion treatability.

Si: more than 0.7%

Si is an element that can contribute to the stabilization of ferrite, increase the strength of steel through solid-solution hardening or improvement in quenching hardenability, and improve formability. A large amount of Si generally results in a high elongation and improved formability but a marked deterioration in chemical conversion treatability. The deterioration of chemical conversion treatability is tolerable at a Si content of 0.7% or less. Thus, in exemplary embodiments of the present invention, the lowest Si content is more than 0.7% at which chemical conversion treatability is previously said to deteriorate markedly. The lowest Si content is preferably 1% or more. The Si content of 1% or more still has a problem in the chemical conversion treatability of steel materials in the prior arts. Even at such a Si content that may previously result in a marked deterioration in chemical conversion treatability, however, the present invention can provide a member having excellent chemical conversion treatability. Although the highest Si content in the present invention is not particularly limited, the Si content is preferably 2.5% or less in terms of the quality of a material.

The adverse effects of Si on chemical conversion treatability result from the surface enrichment of an oxide mainly composed of Si and do not result from the surface enrichment of Si alone. The surface enrichment of an oxide mainly composed of Si can occur during hot rolling. In this case, subsequent pickling treatment can partly remove the oxide. Annealing also causes surface enrichment in an annealing furnace. It is difficult to control the degree of enrichment of an oxide mainly composed of Si in the manufacture of steel sheets.

Mn: 0.8% or more

In the same manner as in C, Mn is an element that can increase the strength of steel through solid-solution hardening and improvement in quenching hardenability. In exemplary embodiments of the present invention, the Mn content of 0.8% or more is required to ensure a desired high strength. Furthermore, Mn can fix S in steel as MnS, thereby making S harmless. Thus, the Mn content is limited to 0.8% or more. The Mn content is preferably 1.5% or more to ensure the tensile strength of 780 MPa or more. An excessive amount of Mn of more than 5% results in a marked decrease in ductility. Thus, the Mn content is preferably limited to 5% or less.

In addition to the basic components described above, a composition further containing 0.1% or less Al and 0.010% or less N is preferred.

Al: 0.1% or less

Al is an element that can act as a deoxidizer and fix N as AlN, thereby preventing adverse effects of N. Such effects are significant at an Al content of 0.01% or more. The Al content of more than 0.1% results in an increase in the amount of Al-based inclusion, thereby impairing the cleanliness of steel. Thus, the Al content is limited to 0.1% or less, preferably 0.06% or less.

N: 0.010% or less

In the same manner as in C, N is an element that can increase the strength of steel by solid solution. A large amount of N, however, results in a decrease in ductility. Thus, the N content is preferably limited to 0.010% or less, more preferably 0.0050% or less.

In addition to the components described above, one or at least two selected from 0.03% or less Ti, 0.1% or less Nb, and 0.1% or less V, and/or one or at least two selected from 1% or less Cr, 1% or less Mo, 1% or less Ni, 1% or less Cu, and 0.01% or less B, and/or one or two selected from 0.1% or less Ca and 0.05% or less REM may be contained if necessary.

One or at least two selected from 0.03% or less Ti, 0.1% or less Nb, and 0.1% or less V

Ti, Nb, and V are elements that can form carbonitrides, prevent the coarsening of crystal grains, and contribute to high strength through precipitation hardening. One or at least two of them may be appropriately used. Such effects can be observed at a Ti content of 0.01% or more, a Nb content of 0.005% or more, or a V content of 0.01% or more. However, a Ti content of more than 0.03%, a Nb content of more than 0.1%, or a V content of more than 0.1% results in a marked decrease in ductility. Thus, if present, the Ti content is preferably 0.03% or less, the Nb content is preferably 0.1% or less, and the V content is preferably 0.1% or less. More preferably, the Ti content is 0.025% or less, the Nb content is 0.05% or less, and the V content is 0.05% or less.

One or at least two selected from 1% or less Cr, 1% or less Mo, 1% or less Ni, 1% or less Cu, and 0.01% or less B

Cr, Mo, Ni, Cu, and B are elements that can contribute to an increase in the strength of steel through solid-solution hardening or improvement in quenching hardenability. One or at least two of them can be appropriately used. Such effects can be observed at a Cr content of 0.03% or more, a Mo content of 0.02% or more, a Ni content of 0.03% or more, a Cu content of 0.02% or more, or a B content of 0.001% or more. Cu can contribute to improvements in corrosion resistance and resistance to delayed fracture. However, a Cr content of more than 1%, a Mo content of more than 1%, a Ni content of more than 1%, a Cu content of more than 1%, or a B content of more than 0.01% adversely affects weldability and the integrity of an electric resistance weld. Thus, if present, the Cr content is preferably 1% or less, the Mo content is preferably 1% or less, the Ni content is preferably 1% or less, the Cu content is preferably 1% or less, and the B content is preferably 0.01% or less. More preferably, the Cr, Mo, Ni, or Cu content is 0.5% or less, and the B content is 0.005% or less.

One or two selected from 0.1% or less Ca and 0.05% or less REM

Ca and REM are elements that can control the morphology of an inclusion and contribute to an improvement in ductility. One or two of them may be appropriately used. Such effects are significant at a Ca content of 0.002% or more or a REM content of 0.02% or more. However, a Ca content of more than 0.1% or a REM content of more than 0.05% results in an excessive amount of inclusion, thus lowering ductility. Thus, if present, the Ca content is preferably 0.1% or less, and the REM content is preferably 0.05% or less. More preferably, the Ca content is 0.01% or less, and the REM content is 0.01% or less.

The remainder other than the components described above includes Fe and incidental impurities. Allowable incidental impurities are 0.02% or less P and 0.005% or less S. A P content of more than 0.02% or a S content of more than 0.005% results in a marked deterioration in toughness and weldability.

A steel material having the composition described above used as a raw material in the present invention may have any structure. A steel material having any structure, such as a ferrite-based structure, a martensite-based structure formed by quenching treatment during an annealing process after cold rolling, or a structure containing retained austenite or bainite, may be used as a steel material for use in the present invention. A steel material for use in the present invention as a raw material may be manufactured by any method. Any steel material manufactured by any method, such as a hot-rolled steel sheet, a cold-rolled steel sheet, or a steel tube, whether annealed or not, is applicable as a raw material in the present invention.

Since the Si content of a steel material in exemplary embodiments of the present invention is more than 0.7%, it is assumed that an oxide mainly composed of Si causing deterioration in chemical conversion treatability is present on the steel surface. An oxide mainly composed of Si on the steel surface is formed during a hot rolling process and/or an annealing process.

For example, in sheet applications that often require processing before use, hot-rolled sheets are shipped after pickling. Steel sheets in which an oxide mainly composed of Si formed on their surfaces is partly removed by pickling are used as raw materials for the manufacture of members. It is probably difficult to use steel sheets with mill scale as raw materials for processing that are subjected to chemical conversion treatment.

Cold-rolled sheets are manufactured by pickling of hot-rolled sheets, subsequent cold rolling, and optionally annealing, such as continuous annealing. During the annealing process, such as continuous annealing, an oxide mainly composed of Si is again formed on the surface in an environment within an annealing furnace. The formation of an oxide mainly composed of Si depends greatly on the environment within an annealing furnace, that is, the atmosphere within the furnace (such as the dew point), the line speed, the timing of line stop in upstream and downstream processes, and unusual situations, such as the opening of the furnace, and cannot be fully estimated from the process parameters. Even steel sheets having different degrees of Si enrichment are applicable as raw materials in the present invention.

In exemplary embodiments of the present invention, when a high-strength steel material preferably having the composition described above is processed into a member (high-strength member) having a predetermined shape, the processing is controlled such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 5% or more as nominal strain.

The processing of a steel material is inevitably accompanied with strain on the steel material. The strain can be assessed, for example, in three directions perpendicular to one another (x, y, and, z directions), that is, two in-plane directions intersecting at right angles and one thickness direction perpendicular to the two in-plane directions.

In exemplary embodiments of the present invention, the surface of a steel material to be processed is processed such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 5% or more as nominal strain. When the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is less than 5%, a marked improvement in chemical conversion treatability cannot be expected. Preferably, the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 7% or more. The predetermined direction is preferably the largest deformation direction. Alternatively, the predetermined direction may be a predetermined direction and another direction perpendicular to the predetermined direction, and the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is the sum total of absolute surface strains in the two directions perpendicular to each other, that is, the total of the sum total of absolute surface strains in the predetermined direction and the sum total of absolute surface strains in the other direction perpendicular to the predetermined direction.

In the processing of a steel material in accordance with exemplary embodiments of the present invention, processing strain applied in the thickness direction of the steel material is not considered. This is because the chemical conversion treatability of a member principally depends on the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing.

In the case that a steel material used as a raw material has surface strain in its manufacture, the processing conditions of the steel material may be controlled such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing in combination with the sum total of absolute surface strains applied in its manufacture is 5% or more as nominal strain. Even when the total of the absolute processing strains (surface strains) applied in the manufacture of a steel material and the absolute processing strains (surface strains) applied after the manufacture is 5% or more, a member having excellent chemical conversion treatability can be produced.

The term “excellent chemical conversion treatability”, as used herein, means that the structure of iron-zinc phosphate crystals and corrosion resistance after coating are both good. More specifically, iron-zinc phosphate crystals are dense and uniform grains and have a structure containing no crystal-free area, and a coating film after coating exposed to a corrosive environment has excellent corrosion resistance such that a phenomenon called an alkali blister or a puff at cathode area occurs insignificantly. The phenomenon called an alkali blister or a puff at cathode area is a phenomenon on the precondition of a wet coating film environment in which a crosscut area 2 acts as an anode, a portion that finally becomes a puff acts as a cathode, and the anode and the cathode constitute a cell with a coating film interposed therebetween.

The term “uniform grains” in the context of the iron-zinc phosphate crystal structure refers to an average grain diameter ±20% or less for seemingly uniform grains and, for apparently a mixture of coarse grains and fine grains, means that the size of the coarse grains is not more than three times the size of the fine grains.

The term “no crystal-free area” in the context of the iron-zinc phosphate crystal structure means that no “crystal-free area” can be observed at a magnification ratio of 1000 in two or more visual fields in random portions near the center of a test sample except abnormal portions. The term “crystal-free area” generally refers to an area having no iron-zinc phosphate crystal. However, observation under magnification shows that there are a portion seemingly free of iron-zinc phosphate crystals and a portion containing a small number of much smaller iron-zinc phosphate crystals than neighboring iron-zinc phosphate crystals at a very low density. Thus, the term “crystal-free area”, as used herein, means that no iron-zinc phosphate crystal is formed in an area of more than three times the iron-zinc phosphate crystal grain size (diameter) for uniform iron-zinc phosphate crystal grains (an average grain diameter ±20% or less) and, for iron-zinc phosphate crystals containing a mixture of coarse grains and fine grains, means that no iron-zinc phosphate crystal is formed in an area of more than five times the size (diameter) of the coarse grains.

The corrosion resistance after coating is examined and evaluated as described below.

As illustrated in FIG. 8( a), a test specimen 1 includes a target area for a corrosion test surrounded by a masking tape 3. The target area (exposed portion) is at least 30 mm×100 mm. In the case of a steel tube, the test specimen 1 is a halved tube. If a steel tube for the test specimen 1 is too small to satisfy the exposed area described above, two or more test specimens 1 may be used for the evaluation. When the processing strain cannot be uniformly applied throughout the entire test specimen for the corrosion test, the size of the test specimen 1 is adjusted such that the exposed portion of the test specimen 1 includes a processed portion.

The test specimen 1 is subjected to chemical conversion treatment and is coated with a film by electrodeposition coating. A crosscut 2 is then formed on a surface of the test specimen 1. After the corrosion test is performed, a swollen width 4 on one side of the crosscut 2 is measured. The swollen width 4 smaller than a predetermined value indicates excellent corrosion resistance after coating. The excellent chemical conversion treatability of the test specimen 1 may also be determined by simultaneously subjecting mild steel (SPCC) to the corrosion test and confirming that the corrosion resistance of the test specimen 1 is equivalent to or better than the corrosion resistance of the mild steel with the limits of error taken into account and that a normal portion other than the crosscut 2 and a portion adjacent to the crosscut 2 has no pimple, blister, swelling, or exposure of the substrate. The corrosion conditions for the corrosion test may be any corrosion test, such as a hot salt dip test, a salt spray test (SST), or a cyclic corrosion test.

A surface strain applied in each process step of the processing is preferably measured using scribed circles (SCs) 6 (hereinafter referred to as SCs) transferred to a surface of a steel material to be processed. For example, after a steel material on which the scribed circles (SCs) 6 are transferred as illustrated in FIG. 3 is processed, the length of one of the SCs 6 of interest is measured, for example, with a thread-like flexible scale and is converted into surface strain. Since the SCs 6 transferred can be erased with an organic solvent, the chemical conversion treatability of the portion the surface strain of which was measured can be evaluated.

In the present invention, the calculation of a surface strain applied in each process step of the processing is based on its absolute value without considering the tensile and compressive directions. Thus, the present invention utilizes, as a measure, the surface strain applied in each process step of the processing, that is, the sum total of absolute surface strains. In exemplary embodiments of the present invention, the surface strain applied in each process step of the processing is not true strain but nominal strain. This is based on the finding that the chemical conversion treatability principally depends on the sum total of absolute values of nominal strains each applied in individual process steps of the processing.

The term “processing”, as used herein, includes the stamping, the bending, the drawing, and their combined processing of sheet materials, the bending, the compression, and their combined processing of bar materials, and the hydroforming, the bending, the pipe expanding, and their combined processing of tubing materials.

The present invention will be described in detail below with reference to examples.

EXAMPLES

Steel materials No. A to No. G were used. These steel materials had compositions shown in Table 1 and tensile properties and chemical conversion treatability shown in Table 2. These steel materials were continuous annealed (CAL) cold-rolled steel strips (cold-rolled and continuously annealed sheets), pickling-treated hot-rolled steel strips (hot-rolled pickled sheets), and electric-resistance-welded steel tubes (welded steel tubes) manufactured by electric-resistance welding using the cold-rolled steel strips and the hot-rolled steel strips as mother sheets. As illustrated in FIG. 9, a process for manufacturing electric-resistance-welded tubes includes continuous steps of rewinding a coil of a steel strip 7, straightening the sheet with a leveler 8, forming a tube in a roll-forming process 9 and an electric-resistance-welding process 10, and performing a diameter-reduction-based straightening process 11 with a sizer, and subsequently cutting into product tubes having predetermined dimensions with a cutting machine 12. This process for manufacturing electric-resistance-welded tubes produces at least a circumferential surface strain. After the diameter-reduction-based straightening process 11, a straightening process 13 using a straightening machine may be performed. Tables 3-1 to 3-4 show surface strains applied to an outer surface layer of these electric-resistance-welded steel tubes.

Test specimens sampled from these steel materials were subjected to various types of processing, forming members of various shapes. Test specimens sampled from these members were evaluated for chemical conversion treatability.

The processing was performed in the following four processing patterns.

(1) Processing Pattern a

Processing in a processing pattern a included the bending of a test specimen (150 mm width×300 mm length) as illustrated in FIG. 4. In this case, the processing was simple bending using a core material 15 at the center of bending (FIG. 4( a)) or without using the core material 15 (FIG. 4( b)). The bending angle was altered to change strain. In the measurement of strain, the scribed circles (SCs) 6 as illustrated in FIG. 3 were transferred to both sides of a sheet beforehand, the sheet was subjected to predetermined processing, and, after settling in a final shape, the sizes of the SCs 6 on the inside and the outside of the bent portion were measured with a thread-like flexible scale in the bending direction. The sizes of the SCs 6 were then converted into surface strains. The strain in a direction (width direction) perpendicular to the bending direction was not measured because it was negligible. Thus, the strain can be calculated by (diameter in the bending direction _(after straightening)−diameter in the bending direction _(before straightening))/diameter in the bending direction _(before straightening). The SCs 6 of 5 mm diameter were used as standards and three or more SCc including deformed portions were measured for the calculation of surface strain. However, more than 10 SCs may include an undeformed portion, resulting in inappropriate measurement of surface strain. For a smaller bend radius, smaller SCs 6 were used.

(2) Processing Pattern b

Processing in a processing pattern b included the stretch forming of a test specimen 16 using a flat-bottom punch 17 as illustrated in FIG. 5. The test specimen 16 had 200 mm width×200 mm length as a standard blank size. The length of one side of the blank size was adjusted depending on surface strain applied to change the size of the test specimen 16. Furthermore, the stretch height h and the like were adjusted to change surface strain. In the measurement of surface strain, scribed circles (SCs) 6 as illustrated in FIG. 3 were transferred to a surface of a test specimen 16 in advance. After a surface opposite the surface to which the SCs 6 were transferred was subjected to predetermined processing, the sizes of the SCs 6 on the surface not contacted with the punch were measured approximately at the center of the test specimen in an X direction and a Y direction perpendicular to the X direction with a thread-like flexible scale. The sizes of the SCs 6 were then converted into surface strains. The SCs 6 measured were evaluated in a direction parallel to a side of the test specimen 16. A portion for the evaluation of surface strain was cut out and was evaluated for chemical conversion treatability.

(3) Processing Pattern c1

In processing in a processing pattern c1, a test sheet was processed into an actual member shape (a processed product) illustrated in FIG. 6. In the measurement of surface strain, scribed circles (SCs) 6 as illustrated in FIG. 3 were transferred to both surfaces of a test sheet in advance. After predetermined processing, the sizes of the SCs 6 on the outside and the inside of the processed product were measured in an X direction and a Y direction perpendicular to the X direction with a thread-like flexible scale. The sizes of the SCs 6 were then converted into surface strains. The number of SCs 6 measured was at least three in a uniformly processed wide region. In a region on which strain was locally concentrated, the number of SCs 6 measured was at least three including a portion (a corner) on which processing was locally concentrated. However, more than 10 SCs may include an unprocessed portion, resulting in inappropriate measurement of surface strain. A portion for the evaluation of surface strain was cut out and was evaluated for chemical conversion treatability.

(4) Processing Pattern c2

In processing in a processing pattern c2, a test specimen (a steel tube) was processed into an actual member shape (a processed product) illustrated in FIG. 7. In the measurement of surface strain, scribed circles (SCs) 6 as illustrated in FIG. 3 were transferred to a surface (an outer surface) of a test specimen (a steel tube) in advance. After predetermined processing, the sizes of the SCs 6 on the outside of the processed product were measured in an X direction and a Y direction perpendicular to the X direction with a thread-like flexible scale. The sizes of the SCs 6 were then converted into surface strains (it was assumed that strain on the inside was substantially the same as the strain on the outside). The number of SCs 6 measured was at least three in a uniformly processed wide region. In a region on which strain was locally concentrated, the number of SCs 6 measured was at least three including a portion (a corner) on which processing was locally concentrated. However, more than 10 SCs may include an unprocessed portion, resulting in inappropriate measurement of surface strain. A portion for the evaluation of surface strain was cut out and was evaluated for chemical conversion treatability.

Tables 3-1 to 3-4 show the measurements of surface strain.

A test specimen 1 was sampled from a test specimen thus processed and was evaluated for chemical conversion treatability. In the evaluation of chemical conversion treatability, after the SCs 6 were sufficiently erased with an organic solvent, the test specimen 1 was sampled from a portion in which the surface strain was measured. The test specimen 1 of a sheet material had a size of 70 mm width×150 mm length in the rolling direction. The test specimen 1 of a tubular material was a halved tube having a length in the range of 100 to 150 mm in the rolling direction. For a narrow processed region, a plurality of test specimens 1 were sampled.

The test specimen 1 was then successively subjected to degreasing treatment, water washing, surface conditioning, chemical conversion treatment, and cathodic electrodeposition coating. A test specimen subjected to chemical conversion treatment but not subjected to cathodic electrodeposition coating was also prepared.

In the degreasing treatment, a surface of the test specimen was sprayed with a drug solution SD250HM made by Nippon Paint Co., Ltd. at a temperature of 42° C. for 120 s. In the surface conditioning, the test specimen 1 was immersed in a chemical solution 5N-10 made by Nippon Paint Co., Ltd. for 30 s in a room temperature environment. In the chemical conversion treatment, the test specimen 1 was immersed in a chemical solution SD2800 made by Nippon Paint Co., Ltd. for 120 s at a liquid temperature of 43±3° C., a total phosphoric acid concentration (TA) in the range of 20 to 26, a free acid concentration (FA) in the range of 0.7 to 0.9, and an accelerator concentration (AC) in the range of 2.8 to 3.5 and was baked at 170° C. for 20 min. In the cathodic electrodeposition coating, a coating film having a thickness in the range of approximately 20 to 25 μm was formed using a chemical solution PN-150 gray made by Nippon Paint Co., Ltd. at a liquid temperature of 28° C., an applied voltage of 180 V, and a treating time of 180 s.

In the same manner as illustrated in FIG. 8( a), a crosscut 2 (for a small-diameter tube having an outer diameter of 40 mmφ or less, only one line in the longitudinal direction) was formed on the outer surface and the inner surface of the test specimen 1 subjected to the cathodic electrodeposition coating. The ends of the test specimen 1 approximately 10 mm in width were covered with a masking tape 3. The test specimen 1 was then subjected to a salt dip test (SDT) involving immersion in a 5% NaCl aqueous solution (at a liquid temperature of 55° C.) for 10 days. For a narrow processed region, the crosscut 2 was provided in the processed region (in the vicinity of the center of forming), and the swollen width was measured.

After immersion, a cellophane tape was attached to the test specimen 1 and was then peeled off. As illustrated in FIG. 8( b), the maximum swollen width (one-side) 4 from the crosscut 2 was measured on the inner surface and the outer surface. The chemical conversion treatability was determined to be good (OK) when the maximum swollen width (one-side) was 2.5 mm or less. Otherwise, the chemical conversion treatability was determined to be poor (NG).

Furthermore, iron-zinc phosphate crystals on the inner surface and the outer surface of the test specimen 1 subjected to chemical conversion treatment were observed with a scanning electron microscope (magnification ratio: 1000). The chemical conversion treatability was determined to be good (OK) when the iron-zinc phosphate crystals were dense and “uniform grains” with “no crystal-free area”. Otherwise, the chemical conversion treatability was determined to be poor (NG). The definitions of “uniform grains” and “no crystal-free area” were the same as in the basic experiment described above.

Tables 4-1 and 4-2 show the evaluation results of chemical conversion treatability.

Members No. 1, No. 7, No. 15, and No. 22, which are steel materials (steel sheets) serving as raw materials and are described for reference, have poor chemical conversion treatability. On the other hand, members No. 3 to No. 6, No. 9 to No. 10, No. 18 to No. 21, No. 23, No. 25, and No. 27 have improved chemical conversion treatability. In these members, a surface strain applied by processing was a predetermined value (5.0%) or more, or a surface strain applied by processing was less than a predetermined value (5.0%), but the sum total of the surface strain applied by processing and a surface strain applied in the manufacture of a steel material was a predetermined value (5.0%) or more. Members No. 2, No. 8, No. 16, No. 17, No. 24, and No. 26, which have a surface strain less than a predetermined value, show no significant improvement in chemical conversion treatability. The surface strains applied to the inside and the outside of a processed portion are opposite in direction but are the same in absolute value. The chemical conversion treatability is improved on both the inside and the outside. The effects of improving chemical conversion treatability are independent of the surface strain direction.

A member No. 11, which is a steel material (a steel sheet) having a Si content outside the scope of the present invention and is described for reference, has an acceptable level of chemical conversion treatability as a raw material. A comparison of members No. 12 to No. 14, which were processed from such a steel material and experienced surface strain, with the member No. 11 shows that processing slightly improves chemical conversion treatability.

Members No. 28 and No. 34, which are steel materials (steel sheets No. 6 and No. 8) serving as raw materials and are described for reference, have poor chemical conversion treatability. In contrast, a steel material No. 7 (a member No. 29), which was an electric-resistance-welded steel tube manufactured by electric-resistance welding using this steel material (the steel sheet No. 6) as a mother sheet, has improved chemical conversion treatability. In the member No. 29, which is a steel material (the steel material No. 7: steel tube) serving as a raw material and is described for reference, the (circumferential) surface strain applied in the manufacture of an electric-resistance-welded tube is a predetermined value (5.0%) or more. Further application of surface strain to this electric-resistance-welded steel tube by processing further improves chemical conversion treatability.

A steel material No. 9 (a member No. 35), which was an electric-resistance-welded steel tube manufactured by electric-resistance welding using the steel material No. 8 (the member No. 34) as a mother sheet, shows an insufficient improvement in chemical conversion treatability. The (circumferential) surface strain applied in the manufacture of the electric-resistance-welded tube is less than a predetermined value (5.0%). On the other hand, members No. 36 and No. 37 have improved chemical conversion treatability. In the members No. 36 and No. 37, a surface strain was applied to such an electric-resistance-welded steel tube by processing, and the total of the surface strain applied by processing and the (circumferential) surface strain applied in the manufacture of the electric-resistance-welded tube is a predetermined value (5.0%) or more. In contrast, a member No. 38 showed no significant improvement in chemical conversion treatability. In the member No. 38, although a surface strain was applied by processing, the total of the surface strain applied by processing and the (circumferential) surface strain applied in the manufacture of the electric-resistance-welded tube was less than a predetermined value (5.0%).

In accordance with the present invention, a member manufactured using a high-strength steel material having a high Si content of more than 0.7% on the basis of mass percent as a raw material can be a high-strength member having excellent chemical conversion treatability without performing mechanical grinding or chemical pickling treatment. Thus, the present invention has significant industrial advantages. Also in accordance with the present invention, a member having excellent chemical conversion treatability can be manufactured independently of the history of a steel material used as a raw material.

REFERENCE SIGNS LIST

-   -   1: test specimen (for crosscut)     -   2: crosscut     -   3: masking     -   4: maximum swollen width (one-side)     -   5: test specimen (for the presence or absence of crystal-free         area)     -   6: scribed circles     -   7: steel strip     -   8: leveler     -   9: roll-forming process     -   10: electric-resistance-welding process     -   11: diameter-reduction-based straightening process     -   12: tube-cutting machine     -   13: straightening process     -   14: test specimen (for bending)     -   15: core material     -   16: test specimen (for stretch forming)     -   17: flat-bottom punch     -   h: stretch height

TABLE 1 Steel Components (% by mass) No. C Si Mn P S Al N Nb, Ti, V Mo, Cr, Ni, Cu, B Ca, REM A 0.14 1.3 2 0.009 0.001 0.045 0.0025 Nb: 0.01 — — B 0.15 1.7 2.4 0.008 0.002 0.052 0.0035 — — — C 0.09 0.32 2.9 0.01 0.001 0.042 0.0018 Nb: 0.01 — — D 0.14 1.5 1.65 0.007 0.001 0.045 0.0015 — Mo: 0.02 — E 0.09 0.81 2.3 0.009 0.002 0.047 0.0031 Ti: 0.02 Cr: 0.05, Cu: 0.02 Ca: 0.002 F 0.13 1.5 1.7 0.012 0.002 0.045 0.0025 — — — G 0.1 1.8 1.8 0.009 0.001 0.04 0.003 — — —

TABLE 2 Chemical conversion treatability Corrosion resistance Steel Outer Tensile properties of coating film** material Steel Thickness diameter YS TS EI Oxide (Swollen width (one- No. No. Type (mm) (mm) MPa MPa % crystals* side): mm) 1 A Cold-rolled and continuously 1.6 — 725 985 18 NG NG(4.5) annealed sheet 2 B Cold-rolled and continuously 2 — 990 1295 7 NG NG(3.9) annealed sheet 3 C Cold-rolled and continuously 1.4 — 660 1085 15 OK OK(1.6) annealed sheet 4 D Cold-rolled and continuously 1.8 — 820 1190 16 NG NG(4.3) annealed sheet 5 E Cold-rolled and continuously 1.4 — 780 1100 13 NG NG(3.6) annealed sheet 6 F Cold-rolled and continuously 2 — 800 1090 15 NG NG(4.2) annealed sheet 7 F Electric-resistance-welded 2 48.6 945 1130 23 OK OK(2.5) steel tube 8 G Cold-rolled continuously 1.4 — 870 1100 15 NG NG(5.2) annealed sheet 9 G Electric-resistance-welded 1.4 89.1 960 1170 21 NG NG(3.6) steel tube *OK: Uniform grains with no crystal-free area. NG: Other than OK **OK: Swollen width (one-side) is 2.5 mm or less. NG: Other than OK

TABLE 3-1 Surface strain State of steel material applied in processing Surface strain Bending applied in direction Steel manufacture of Processing (X direction) Member material Steel material pattern a (%) No. No. No. State c (%) No. Inside Outside 1 1 A After skin pass rolling 0.4 — —  — 2 1 A After skin pass rolling 0.4 a −2.7 2.8 3 1 A After skin pass rolling 0.4 a −4.7 4.6 4 1 A After skin pass rolling 0.4 a −5.2 5.1 5 1 A After skin pass rolling 0.4 a −7.7 7.7 6 1 A After skin pass rolling 0.4 a −9.9 10.1 7 2 B After skin pass rolling 0.3 — —  — 8 2 B After skin pass rolling 0.3 a —* 2.1 9 2 B After skin pass rolling 0.3 a —* 5 10 2 B After skin pass rolling 0.3 a —* 8 Surface strain applied in processing Direction perpendicular Sum of to bending direction absolute values |c| + |a| + |b| Member (Y direction) b (%) |a| + |b|(%) (%) No. Inside Outside Inside Outside Inside Outside Note 1 — — —  — 0.3 0.3 Reference 2 —* —* 2.7 2.8 3.1 3.2 Comparative example 3 —* —* 4.7 4.6 5.1 5 Example 4 —* —* 5.2 5.1 5.6 5.5 Example 5 —* —* 7.7 7.7 8.1 8.1 Example 6 —* —* 9.9 10.1 10.3 10.5 Example 7 —  —  —  — 0.3 0.3 Reference 8 —* —* —* 2.1 —* 2.4 Comparative example 9 —* —* —* 5 —* 5.3 Example 10 —* —* —* 8 —* 8.3 Example *Not evaluated. **Surface not contacted with core of bending, punch, or press die. ***Estimated value.

TABLE 3-2 Surface strain State of steel material applied in processing Surface strain Bending applied in direction Steel manufacture of Processing (X direction) Member material Steel material pattern a (%) No. No. No. State c (%) No. Inside Outside 11 3 C After skin pass rolling 0.5 — — — 12 3 C After skin pass rolling 0.5 a  —* 4 13 3 C After skin pass rolling 0.5 a  —* 7.3 14 3 C After skin pass rolling 0.5 a  —* 12 15 4 D After skin pass rolling 0.5 — — — 16 4 D After skin pass rolling 0.5 b — 1.5** 17 4 D After skin pass rolling 0.5 b — 1.7** 18 4 D After skin pass rolling 0.5 b — 2.8** 19 4 D After skin pass rolling 0.5 b — 4.0** 20 4 D After skin pass rolling 0.5 b — 5.1** Surface strain applied in processing Direction perpendicular Sum of to bending direction absolute values |c| + |a| + |b| Member (Y direction) b (%) |a| + |b|(%) (%) No. Inside Outside Inside Outside Inside Outside Note 11 — — — — 0.5 0.5 Reference 12  —*  —*  —* 4  —* 4.5 Comparative example 13  —*  —*  —* 7.3  —* 7.8 Comparative example 14  —*  —*  —* 12  —* 12.3 Comparative example 15 — — — — 0.5 0.5 Reference 16 — 1.6** — 3.1 — 3.6 Comparative example 17 — −0.9** — 2.6 — 3.1 Comparative example 18 — 2.7** — 5.6 — 6.1 Example 19 — −1.1** — 5.1 — 5.6 Example 20 — 0.1** — 5.2 — 5.7 Example *Not evaluated. **Surface not contacted with core of bending, punch, or press die. ***Estimated value.

TABLE 3-3 Surface strain State of steel material applied in processing Surface strain Bending applied in direction Steel manufacture of Processing (X direction) Member material Steel material pattern a (%) No. No. No. State c (%) No. Inside Outside 21 4 D After skin pass rolling 0.5 b  —   6.8** 22 5 E After skin pass rolling 0.5 — —  — 23 5 E After skin pass rolling 0.5 c1 −2.5*** −2.5  24 5 E After skin pass rolling 0.5 c1 —* 1.2 25 5 E After skin pass rolling 0.5 c1 —* 4.2 26 5 E After skin pass rolling 0.5 c1 —* 0.7 27 5 E After skin pass rolling 0.5 c1 —* 2.2 28 6 F After skin pass rolling 0.5 — —  — 29 7 F After welded 5.0 (circumferential) — —  — 30 7 F After welded 5.0 (circumferential) c2  1.5*** 1.5 Surface strain applied in processing Direction perpendicular Sum of to bending direction absolute values |c| + |a| + |b| Member (Y direction) b (%) |a| + |b|(%) (%) No. Inside Outside Inside Outside Inside Outside Note 21 —  −1.8 —  8.6 —  9.1 Example 22 —  — —  — 0.5 0.5 Reference 23 3.2*** 3.2 5.7*** 5.7 6.2*** 6.2 Example 24 —* 1.8 —* 3 —* 3.5 Comparative example 25 —* −1 —* 5.2 —* 5.7 Example 26 —* 0.8 —* 1.5 —* 2 Comparative example 27 —* 2.8 —* 5 —* 5.5 Example 28 —  — —  — 0.3 0.3 Reference 29 —  — —  — 5 5 Reference 30 1.7*** 1.7 3.2*** 3.2 8.2*** 8.2 Example *Not evaluated. **Surface not contacted with core of bending, punch, or press die. ***Estimated value.

TABLE 3-4 Surface strain State of steel material applied in processing Surface strain Bending applied in direction Steel manufacture of Processing (X direction) Member material Steel material pattern a (%) No. No. No. State c (%) No. Inside Outside 31 7 F After welded 5.0 (circumferential) c2 0.5*** 0.5 32 7 F After welded 5.0 (circumferential) c2 2.1*** 2.1 33 7 F After welded 5.0 (circumferential) c2 0.7*** 0.7 34 8 G After skin pass rolling 0.5 — — — 35 9 G After welded 2.5 (circumferential) — — — 36 9 G After welded 2.5 (circumferential) c2 2.3*** 2.3 37 9 G After welded 2.5 (circumferential) c2 1.5*** 1.5 38 9 G After welded 2.5 (circumferential) c2 0.8*** 0.8 Surface strain applied in processing Direction perpendicular Sum of to bending direction absolute values |c| + |a| + |b| Member (Y direction) b (%) |a| + |b|(%) (%) No. Inside Outside Inside Outside Inside Outside Note 31 1.3*** 1.3 1.8*** 1.8 6.8*** 6.8 Example 32 1.8*** 1.8 3.9*** 3.9 8.9*** 8.9 Example 33 0.8*** 0.8 1.5*** 1.5 6.5*** 6.5 Example 34 — — — — 0.3 0.3 Reference 35 — — — — 2.5 2.5 Reference 36 2.2*** 2.2 4.5*** 4.5 7.0*** 7 Example 37 2.1*** 2.1 3.6*** 3.6 6.1*** 6.1 Example 38 1.1*** 1.1 1.9*** 1.9 4.4*** 4.4 Comparative example *Not evaluated. **Surface not contacted with core of bending, punch, or press die. ***Estimated value.

TABLE 4-1 Chemical conversion treatability Corrosion resistance of coating film*** Steel (Swollen width (one- Member material Steel Oxide crystals** side): mm) No. No. No. Inside Outside Inside Outside Note 1 1 A NG NG(4.5) Reference 2 1 A NG NG NG(4.2) NG(4.3) Comparative example 3 1 A OK OK OK(2.5) OK(2.4) Example 4 1 A OK OK OK(2.2) OK(2.1) Example 5 1 A OK OK OK(1.5) OK(1.5) Example 6 1 A OK OK OK(1.1) OK(0.9) Example 7 2 B NG NG(3.9) Reference 8 2 B —* NG —* NG(3.0) Comparative example 9 2 B —* OK —* OK(1.9) Example 10 2 B —* OK —* OK(1.4) Example 11 3 C OK OK(1.6) Reference 12 3 C —* OK —* OK(1.5) Comparative example 13 3 C —* OK —* OK(1.4) Comparative example 14 3 C —* OK —* OK(1.4) Comparative example 15 4 D NG NG(4.3) Reference 16 4 D —* NG —* NG(2.8) Comparative example 17 4 D —* NG —* NG(3.1) Comparative example 18 4 D —* OK —* OK(2.2) Example 19 4 D —* OK —* OK(2.4) Example 20 4 D —* OK —* OK(2.2) Example 21 4 D —* OK —* OK(1.4) Example 22 5 E NG NG(3.6) Reference 23 5 E OK OK OK(2.3) OK(2.4) Example 24 5 E —* NG —* NG(2.7) Comparative example 25 5 E —* OK —* OK(1.8) Example 26 5 E —* NG —* NG(2.8) Comparative example 27 5 E —* OK —* OK(2.4) Example *Not evaluated. **OK: Uniform grains with no crystal-free area. NG: Other than OK ***OK: Swollen width (one-side) is 2.5 mm or less. NG: Other than OK

TABLE 4-2 Chemical conversion treatability Corrosion resistance of coating film*** Steel (Swollen width (one- Member material Steel Oxide crystals** side): mm) No. No. No. Inside Outside Inside Outside Note 28 6 F NG NG(4.2) Reference 29 7 F OK OK OK(2.5) OK(2.5) Reference 30 7 F OK OK OK(2.2) OK(2.2) Example 31 7 F OK OK OK(2.3) OK(2.4) Example 32 7 F OK OK OK(2.0) OK(1.9) Example 33 7 F OK OK OK(2.4) OK(2.3) Example 34 8 G NG NG(5.2) Reference 35 9 G NG NG NG(3.6) NG(3.6) Reference 36 9 G OK OK OK(2.1) OK(2.2) Example 37 9 G OK OK OK(2.3) OK(2.4) Example 38 9 G NG NG NG(2.8) NG(2.7) Comparative example *OK: Uniform grains with no crystal-free area. NG: Other than OK **OK: Swollen width (one-side) is 2.5 mm or less. NG: Other than OK 

1. A method for processing a member having excellent chemical conversion treatability, the method comprising: processing a high-strength steel material having a composition containing, on the basis of mass percent, 0.05% or more C, more than 0.7% Si, and 0.8% or more Mn to form the member, and controlling the processing such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 5% or more as nominal strain.
 2. The method for processing a member according to claim 1, wherein the sum total of absolute surface strains in a predetermined direction is the sum total of absolute surface strains applied in two directions intersecting at right angles.
 3. The method for processing a member according to claim 1, wherein conditions for the processing are controlled such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing in combination with the sum total of absolute surface strains applied in the manufacture of the high-strength steel material is 5% or more as nominal strain.
 4. The method for processing a member according to claim 1, wherein the high-strength steel material is a hot-rolled material or a cold-rolled material.
 5. The method for processing a member according to claim 1, wherein the composition contains, on the basis of mass percent, 0.05% or more C, 1% or more Si, and 1.5% or more Mn.
 6. The method for processing a member according to claim 1, wherein the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 7% or more as nominal strain. 